The combustion of solid fuels at power generating and waste disposal facilities commonly produces air pollutants such as sulfur dioxide, hydrogen chloride and mercury. In a typical DFGT process, an absorber vessel is located downstream of the combustion process into which the gaseous product of combustion (“flue gas”) and a reagent, such as calcium hydroxide, are directed and intermingled. Pollutants then react with the reagent in the absorber vessel to produce solid particles suspended in the flue gas stream. Substantially all of the solid particles are then removed from the flue gas in a particulate removal device located downstream of the absorber vessel such as a fabric filter or electrostatic precipitator.
In some DFGT systems, water is added to the flue gas concurrently with the reagent to enhance the reaction rate in the absorber vessel. This additional water preferably evaporates fully within the absorber vessel, thereby cooling and humidifying the flue gas. The difference between the dry bulb temperature and adiabatic saturation temperature of the flue gas is thereby reduced during this evaporation phase. This difference may be measured at the absorber vessel exit and is commonly referred to as the “approach to saturation temperature” or simply “approach temperature”. Several important aspects of DFGT process performance such as pollutant emission rate, reagent consumption and corrosion rate have been shown to correlate with approach temperature. Consequently, numerous factors affecting approach temperature have been studied in the prior art.
Recent developments in instrumentation technology and analysis techniques have enabled several new factors affecting approach temperature to be confirmed and quantified, one of which is ambient air ingress to DFGT system components such as the absorber vessel. The absorber vessel is under vacuum during normal DFGT system operation; therefore, ambient air tends to enter the vessel through any open orifice. Means to permit and control ambient air ingress into remote regions of the absorber vessel is a common feature of DFGT systems in the prior art. Ambient air may thus be permitted to enter certain remote regions of the absorber vessel continuously, the intended purpose of which is to suspend and convey particulate matter which may otherwise settle by gravity and accumulate in these regions. Nevertheless, heavy particulate matter may still accumulate in these regions and, as a consequence of opening certain valves to permit the discharge of this accumulated matter, additional ambient air may also be permitted to enter certain regions of the absorber vessel. In both the continuous and intermittent air ingress cases, as the cooler ambient air mixes with the humid flue gas in these regions, the localized gas temperature falls below the adiabatic saturation point causing moisture to condense into fine liquid droplets. The liquid droplets agglomerate with the particulate matter to form a semi-solid adhesive mixture that settles and accumulates on the absorber vessel and downstream component surfaces. This accumulation may place unanticipated loads on DFGT system structural components and disturb or impede flue gas flow, thereby interfering with particulate removal and increasing induced draft fan power consumption.
Furthermore, it is now recognized that moisture condensation stemming from the cooling effect of ambient air ingress substantially accelerates corrosion in DFGT systems. The condensed liquid entrained with the solid particles is typically a saturated aqueous salt solution. The corrosion of surfaces onto which this solution is deposited may compromise structural integrity, necessitate costly repairs and reduce the useable life of the DFGT system.
Finally, in addition to the cooling effect of ambient air ingress, the ambient air humidity is also substantially lower than the average flue gas humidity within the absorber vessel. Therefore, during periods of ambient air ingress, this humidity difference further tends to destabilize approach temperature with consequent adverse effects on pollutant emission rate, reagent consumption and particulate removal efficiency.